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Why knots are not recommended in Amsteel or similar lines

I've seen and been involved in many discussions in the time I've been on the forum about Amsteel. In many of them the idea of knots comes up. The general consensus is that it's not a good idea but I wasn't seeing any hard data.

Today I was looking for something else and found this paper published by the Oregon Occupational Safety and Heath Administration. On page 17 (p22 of PDF) there is a table showing destructive testing data of several different knots. The numbers are worse than I ever recall anyone speculating. The worst knot tested was an improved (tucked) half blood knot which broke at 20% of catalog minimum. The best was a cow hitch (Lark's Head) which retained and average of 58%.

The take away point for me is that the average of their testing showed that Amsteel only retains and average of 32% catalog minimum breaking strength when knots are involved.

Ultimately everyone has to hang their own hammock. I just don't want to see anyone get hurt because they lack knowledge of the limitations of the equipment they are working with.

Last edited by Mouseskowitz; 04-27-2013 at 15:27.
Reason: updated terminology

Larks head, Cow hitch and girth hitch are all three basically the same knot other than how they are formed...you achieve the same finished looking product but get there slightly different for each one. End result and strength will be the same though.

I had some extra amsteel left over from some loops I made and just quickly tied the ends together to test some new straps. I managed to sit in the hammock for about 10 minutes until it came undone and the weak point was where the knot was in the amsteel. One bruised elbow & ego later, I was getting up off the ground and making a mental note to not do that again!

... there is a table showing destructive testing data of several different knots. The numbers are worse than I ever recall anyone speculating. The worst knot tested was an improved (tucked) half blood knot which broke at 20% of catalog minimum. The best was a cow hitch (Lark's Head) which retained and average of 58%.

You are correct. Knots weaken ropes considerably. 50% is a rule of thumb, but can go up to 80%, yes.

But don't forget, hardware can weaken ropes just as much or more. A really sharp bend over an edge or hardware with a small diameter can be worse than a bad knot. Even dynamic forces can permanently weaken a rope.